PSI - Issue 2_A

3780 Enrico Salvati et al. / Procedia Structural Integrity 2 (2016) 3772–3781 Author name / Structural Integrity Procedia 00 (2016) 000–000 9 produced by the OL ( � ���� ) resulted to be less than one third smaller of the one at R=0.1. This leads us to think that different crack propagation and retardation mechanisms were operating in the two experiments. The two predominant causes of crack retardation are thought to be the plasticity induced crack closure and the presence of compressive residual stress ahead of the crack tip. In the particular case of very high loading ratio it has been remarked that the crack closure effect may not be present, Borrego (2003). The absence of crack closure after OL at R=0.7 provides an explanation of the difference between the two sets of results. Indeed, the model does not account for the two mechanisms separately, and the reduction of retardation due to high R cannot be considered in the model. Once the Walker-Wheeler model coefficients have been calibrated, the newly formulated model can be used for the prediction of the OL effect on crack retardation at different load ratios and different OL levels. It is important to note that the coefficients evaluate for the Wheeler model need to be interpolated for obtaining a more reliable prediction at load ratios R that differ from the two analysed in this paper (i.e. R=0.1 and R=0.7). Obviously, further experimental points may help improve the statistical coverage and thus the reliability of the model, especially at loading conditions largely different from the two treated here. 5. Conclusion The fatigue crack growth response of Magnesium alloy AZ31b subjected to CGP severe plastic deformation was extensively analysed in the present work. The FCGR comparison between the parent material and the CGP processed material, both tested at the load ratio R=0.1 revealed no substantial differences. This indicates that no significant alteration occurred in the fatigue resistance with the application of the thermo-mechanical treatment. At the load ratio of R=0.1 the fatigue crack growth threshold was found to be �� �������� � �����√� . Further tests at the loading ratio R=0.7 showed a reduced threshold of �� �������� � ������√� . The fitting of experimental data with the Paris coefficients showed nearly the same FCGR curve slope for the two loading ratios. Fatigue tests conducted with the introduction of an OL revealed the retardation behaviour of the material. In both fatigue baseline situations crack propagation showed retardation, and in the case of R=0.7 the crack was even arrested for 2000 cycles. A single test was also performed where an UL was applied at the R=0.1 baseline fatigue test. This showed crack acceleration with a quick restoration of the original steady-state FCGR. However, the effect that UL had on the modification of FCGR was less prominent than the OL. The fatigue crack growth behaviour was modelled using the combination of the Walker model that accounts for material sensitivity to mean stress, and a modified Wheeler model for the crack retardation prediction of crack delay after the occurrence of OL. This model can be used for the prediction of FCGR at different load and overload ratios upon the interpolation of the fitted coefficients. To evaluate the physical meaning and the values of the model parameters, further investigations are required to understand the precise role of crack closure and residual stress in crack retardation. Additionally, further studies may be addressed at the description of the crack arrest behaviour in the present model, to endow this modelling approach with this capability. Acknowledgements AMK acknowledges funding received by MBLEM through EU FP7 project iSTRESS (604646).

References

Avedesian M.M., Baker H., 1999. Magnesium and Magnesium Alloys, M., ASM International, Materials Park. Borrego L.P., Ferreira J.M., Pinho da Cruz J.M., Costa J.M., 2003. Evaluation of overload effects on fatigue crack growth and closure. Eng. Fracture Mechanics 70,1379–1397 Chen W., Zhang W., Qiao Y., Miao Q., Wang E., 2016. Enhanced ductility in high-strength fine-grained magnesium and magnesium alloy sheets processed via multi-pass rolling with lowered temperature. Journal of Alloys and Compounds 665, 13-20. Dowling N. E., Calhoun C.A., Arcari A., 2009. Mean stress effects in stress-life fatigue and the Walker equation. Fatigue and Fracture of Engineering Materials and Structures. 32(3), 163-179 Duran J. A. R., Boloy R. M., Leoni R. R., 2015. Some remarks on the engineering application of the fatigue crack growth approach under nonzero mean loads. Front. Mech. Eng., 10(3), 255–262.